1. Field of the Invention
This invention relates to a motor device having a dynamic-pressure fluid bearing, and more particularly, to the temperature control of the bearing.
2. Description of the Related Art
A deflection scanning device provided, for example, in a laser-beam printer or the like includes a rotating mirror for reflecting a light beam emitted from a light source. Initially, the bearing used for receiving a rotating shaft supporting the rotating mirror was a ball-bearing-type bearing. However, a ball bearing not only has difficulty in rotating at high speed due to the generation of vibration and noise, but also exhibits a backlash between the balls of the bearing and the inner and outer rings supporting the balls; in addition, a mirror surface of the rotating mirror tilts, that is, a so-called tilt of the surface occurs. In order to solve such problems, dynamic-pressure fluid bearings have been developed.
FIG. 6 shows a cross section of a motor for deflection scanning using such a dynamic-pressure fluid bearing. In FIG. 6, a rotating polygon mirror 1, which is a kind of rotating mirror, is mounted on a rotor 2, having the shape of a sleeve and having a cavity in its center, by a mounting flange 3. A driving magnet 4 is fixed to the rotor 2, and is rotatably driven by a stator coil 5, which is a driving coil provided around the circumference of the driving magnet 4 facing it. On an apex portion of the rotor 2, there is provided a thrust plate 6 for generating thrust pressure for an apex portion of a fixed shaft, which will be described later. A fine hole 7 is perforated in the center of the thrust plate 6.
On the other hand, a fixed shaft 8 is inserted within the cavity of the rotor 2, and dynamic-pressure generation grooves 9 are formed on the outer circumference of the fixed shaft 8. The fixed shaft 8 is fixed to a cup-like motor case 10. A cap 10a is fitted to an upper opening end of the motor case 10, and seals up the inside of the deflection scanning motor.
Heretofore, the fluid used in such a dynamic-pressure fluid bearing is a gas, such as air or the like. That is, such a dynamic-pressure fluid bearing has been developed for the purpose of being used at a relatively high number of revolutions of 10,000 rpm or more, and a dynamic-pressure fluid bearing using a gas is suitable for this purpose. However, a dynamic-pressure fluid bearing using gas, such as air or the like as a fluid has disadvantages; these include difficulty in obtaining high precision in the processing of a bearing portion of the motor, and permitting the introduction of dust in the motor, which, even in small amounts, is not allowed.
In order to solve these problems, dynamic-pressure fluid bearings using oil as a fluid have been developed. The oil dynamic-pressure fluid bearings are mostly used as bearings of cylinder heads of VCR's (video cassette recorders) or bearings for turntables of record players, and have been used in fields requiring a relatively small number of revolutions per minute.